Surfaces of optical components in a chamber of an instruments using Extreme Ultraviolet (EUV) light can become contaminated with carbon-containing or oxygen-containing matter resulting in a reduced optical throughput. Various techniques for removal of such contamination are known.
According to one approach, radio frequency (RF) or direct current (DC) discharge devices produce a plasma to generate hydrogen ions to react with the contaminants on surfaces of the optical components and generate a volatile material that disassociates from the surface. Disadvantages of such approaches include: (1) uncontrolled and indiscriminate acceleration of ions in the direction of the optical component causing damage due to ion sputtering, (2) introduction of additional contamination into the instrument due to sputtering of an electrode used to produce the RF or DC voltages, (3) the ions generated are dispersed such that many optical components in a chamber of an apparatus are exposed to the ions, and (4) electrical disturbances arising from the RF and DC devices make them inappropriate for use with lithographic instruments.
According to another approach, a hot filament is used to produce hydrogen radicals (also referred to herein as atomic hydrogen). The radicals react with the contaminants on surfaces of the optical components thereby forming volatile products such as CH4 and H2O that disassociate from the surface.
Disadvantages of such approaches include: (1) thermal disturbances arising from the use of heated filaments (which can reach temperatures of 1500° C.) causing thermal disturbances in an instrument that requires tight temperature control, and (2) evaporation of filament material can cause deposits on optical surfaces possibly causing irreversible contamination of the optical surface.
Accordingly, there remains a need for improved cleaning of optical component in optical instruments.